The invention relates to data communication networks. In particular it relates to packet-switching networks and to the aspect of such networks interworking.
Definitions of some used terms and abbreviations
(STALLNDGS, William, Data and Computer Communications; Macmillan Publishing Company; 1991; and DE PRYCKER, Martin, Asynchronous Transfer Mode: Solution for broadband ISDN; Ellis Horwood series in computer communications and networking; 1991; both herein incorporated by reference)
ISDN: Integrated Services Digital Network;
N-ISDN: Narrowband-ISDN;
SS7: Signalling System Number 7 is a layered set of protocols that is used for control communication internal to a digital network, e.g. an N-ISDN, and provides facilities for establishing, maintaining and terminating connections. It comprises in total four levels;
N-ISLTD: N-ISDN User Part and the fourth level of SS7. It provides for the control signallin needed in an N-ISDN to deal with N-ISDN subscriber calls and related functions;
MTP: Message Transfer Part, lower three levels of SS7, provides a reliable but connectionless service for routing messages through the SS7 network, whereby
MTP1 is the Signalling data link and the first level of SS7;
MTP2 is the Signalling link and the second level of SS7. This level is specified in the ITU-T Recommendation Q.703 (03/93) and is herein incorporated by reference. According to the ITU-T Recommendation Q.703 (03/93) the signalling link functions, together with a signalling data link as bearer, provide a signalling link for reliable transfer of signalling messages between two directly connected signalling points. Signalling messages delivered by superior hierarchical levels are transferred over the signalling link in variable length signal units. A signal unit is constituted of a variable length signalling information field which carries the information generated by a user Part and a number of fixed length fields which carry information required for message transfer control. In the case of link stanus signal units LSSU, the signalling information field and the service information octet is replaced by a status field which is generated by the signalling link terminal. There are three types of signal unit, i.e. the message signal units MSU, link status signal units LSSU and fill-in signal units FISU. The signalling link functions comprise signal unit delimitation, signal unit alignment, error detection, error correction, intitial alignment, signalling link error monitoring and flow control. All these functions are coordinated by the link state control;
MTP3 is the Sionallin, network and the third level of SS7
HDLC: HDLC uses synchronous transmission. All transmissions are in frames, and a single frame format suffices for all types of data and control exchanges. The frame has the following fields: Flag, Address; Control; Information; Frame check sequence (FCS); and Flag. Bit stuffing is a procedure which is used for providing data transparency.
B-ISDN: Broadband-ISDN is a service or system requiring transmission channels of supporting rates greater than the primary rates;
B-ISUP: B-ISDN User part;
ATM: Asychronous Transfer Mode (protocol) is a transfer mode solution for implementing a B-ISDN, comprising three layers defined as the physical layer PHY which mainly transports information;
the ATM layer which mainly performs switching/routing and multiplexing; and
the ATM adaptation layer (AAL) which is mainly responsible for adapting service information to the ATM stream;
ST: Signalling terminal;
ET: Exchange terminal;
IWF/IWU: Interworking function/Interworking unit;
NNI: Network to Network interface;
Node: to which stations attach, is the boundery of a communication network, e.g. a B-ISDN network, and the node is capable of transferring data between pairs of attached stations.
Both information and parameters are sent from and received by an N-ISDN using the SS7 protocol and are furthermore transmitted in a HDLC (High Level Data Link Control) based frame format, e.g. HDLC or LAP-D. The SS7 and the HDLC based frame format are well known in the art. A B-ISDN, however, sends and receives information and parameters using an ATM protocol which is a specific packet oriented transfer mode based on fixed length cells. The ATM protocol is well known in the art. The difficulty in sending data from one type of network to another resides in the use of different protocols and data formats, frames or cells, required for these protocols.
Special interworking units/functions IWU/IWF have been developed for solving the problem of interworking between N-ISDN and B-ISDN. The interworking is performed either by an Interworking Unit, IWU which is a unit separate from the B-ISDN or an Interworking Function, IWF which is an integral part of the B-ISDN. IWU/DV are specified in the ITU-T Recommendation I.580 from 03.93: xe2x80x9cGeneral Arrangements for Interworking between B-ISDN and 64 kbits/s based ISDNxe2x80x9d. Accordingly a standard NNI is the interface between the 64 kbits/s ISDN, i.e. the N-ISDN. and the IWU/IWF and between the B-ISDN and the IWU/IWF. EP-A-0 581 087 discloses an N-ISDN and a B-ISDN interworking by means of an IWU. The advantage of IWU/IWF is that they are standardised. However, they need a large processing capacity. They are furthermore highly dependent on market adaptations. This entails considerable costs.
It is an object of the invention to provide an interworking function which needs less processing capacity.
It is a further object of the invention to provide an interworking function which is less dependent on the market adaptations.
It is yet another object of the invention to provide an interworking function which is low in cost.
These and other objects and advantages are obtained according to the invention as disclosed in independent claim 1 and claim 11. Preferred embodiments of the invention are given in the dependent claims.